Real Estate and Lending Activities - Schedule of Gross Assets by Geographic Areas (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 5,710,395		$ 5,609,351
Assets, Total [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 5,662,830	$ 6,035,922
Percentage of Total Gross Assets	100.00%	100.00%
Assets, Total [Member] | Texas [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,077,738	$ 1,060,990
Percentage of Total Gross Assets	19.00%	17.60%
Assets, Total [Member] | California [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 547,082	$ 547,085
Percentage of Total Gross Assets	9.70%	9.10%
Assets, Total [Member] | All Other States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 2,588,647	$ 3,047,204
Percentage of Total Gross Assets	45.70%	50.40%
Assets, Total [Member] | Other Domestic Assets [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 185,394	$ 177,317
Percentage of Total Gross Assets	3.30%	3.10%
Assets, Total [Member] | United States [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 4,398,861	$ 4,832,596
Percentage of Total Gross Assets	77.70%	80.20%
Assets, Total [Member] | Germany [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,080,381	$ 1,031,039
Percentage of Total Gross Assets	19.10%	17.10%
Assets, Total [Member] | United Kingdom, Italy, and Spain [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 161,880	$ 161,317
Percentage of Total Gross Assets	2.90%	2.70%
Assets, Total [Member] | Other International Assets [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 21,708	$ 10,970
Percentage of Total Gross Assets	0.30%
Assets, Total [Member] | International [Member] | Credit Concentration Risk [Member]
Investment And Revenue From External Customers By Geographic Area [Line Items]
Total Gross Assets	$ 1,263,969	$ 1,203,326
Percentage of Total Gross Assets	22.30%	19.80%